Free Space Path Loss Calculator

Free space path loss (FSPL) is the attenuation of radio frequency (RF) signal power as it travels through free space (vacuum or open air) between a transmitter and receiver with no obstacles. It occurs naturally due to the spreading of electromagnetic waves over increasing distances, even without any physical obstructions. FSPL is a fundamental concept in wireless link budget analysis.

The standard FSPL formula in dB is: FSPL (dB) = 20·log10(d) + 20·log10(f) + 20·log10(4π/c), which simplifies to FSPL (dB) ≈ 20·log10(d_km) + 20·log10(f_MHz) + 32.44. When accounting for antenna gains, the effective path loss becomes FSPL_effective = FSPL_isotropic − G_Tx − G_Rx, where G_Tx and G_Rx are transmit and receive antenna gains in dBi.

FSPL increases by approximately 6 dB every time the distance doubles. This is because path loss is proportional to the square of the distance — a relationship described by the inverse square law. For example, moving from 1 km to 2 km increases FSPL by 6 dB, and moving to 4 km adds another 6 dB.

Higher frequencies experience greater path loss over the same distance. FSPL increases by approximately 6 dB every time the frequency doubles. This is why 5 GHz Wi-Fi has shorter range than 2.4 GHz, and why lower frequency bands (like 700 MHz) are preferred for wide-area cellular coverage.

FSPL is caused by the natural geometric spreading of electromagnetic waves as they propagate outward from an antenna. As distance increases, the same amount of transmitted power is spread over a larger and larger spherical surface area, reducing power density at the receiver. It is not caused by absorption or scattering — those are additional losses that occur in real environments.

For isotropic antennas (gain = 0 dBi on both ends), use the simplified formula: FSPL (dB) = 20·log10(d_km) + 20·log10(f_MHz) + 32.44. Simply enter 0 dBi for both Transmit and Receive Antenna Gain in this calculator, and the result will reflect pure isotropic FSPL.

Shadowing refers to additional signal attenuation caused by large obstacles (buildings, terrain, foliage) between the transmitter and receiver. Unlike free space path loss, which is deterministic and predictable, shadowing is random and location-dependent. In real-world link budgets, a shadowing margin (typically 5–20 dB) is added on top of FSPL to account for these variations.

This calculator accepts distance in kilometers, meters, centimeters, miles, or feet, and frequency in KHz, MHz, or GHz — all converted internally before computation. Antenna gains should be entered in dBi. The output free space path loss is always given in decibels (dB), which is the standard unit for RF link budget calculations.